Aluminum base casting alloys



United States Patent Ofifice 3,322,533 BASE CASTING ALLOYS Fort Wayne, Ind., assignor to William F. Incorporated, Aurora, 111., a corporation of The present invention relates to aluminum base alloys pnmarily used for the production of cast aluminum articles. In particular, the invention is concerned with sand cast alloys, and it relates more particularly to sand cast alloys which are adapted for the development of high strength in the as cast condition, that is, without specific subsequent treatment. The alloys develop even more desirable properties in response to subsequent treatment. Thus, the alloys respond Well to artificial aging alone or they may be solution heat treated and artifically aged to provide for improvement in properties. Finally, the alloys may be allowed to self-age whereby improved properties will develop without specific subsequent treatments.

Prior aluminum base cast-ing alloys employ a wide range of alloying ingredients to effect certain desirable properties. It has been established that such ingredients as zmc, magnesium, chromium, titanium, silicon, boron, zirconium and the like can benefically be included in aluminum base casting alloys in varying proportions. The use of these alloying elements and others in various combinations have provided improved castability in aluminum base alloys in addition to providing improvements in the final properties exhibited by the cast articles.

On the other hand, it has been established that the use of certain alloying elements can be detrimental to the properties of the casting alloys. Elements listed above cannot be used in combination with each other in certain cases and in this connection, the amounts of such elements which are employed can be quite critical. Furthermore, certain other alloying elements such as tin, lead, copper and manganese are considered detrimental for various reasons when present in other than trace amounts.

It is an object of this invention to provide aluminum base casting alloys which include specific alloying elements within relatively narrow composition ranges whereby highly improved properties can be achieved in articles produced with the alloys.

It is a further object of this invention to provide alloys of the type described which are particularly suitable for use in producing sand cast aluminum base articles.

It is an additional object of this invention to provide an aluminum base casting alloy of the type described which specifically excludes certain alloying elements, including some elements conventionally provided by the art in significant amounts, the exclusion of such elements providing casting alloys having extremely satisfactory characteristics.

These and other objects of this invention will appear hereinafter, and it will be understood that the specific examples included herein are provided for purposes of illustration and not by way of limitation.

In accordance with the practice of this invention in one of its forms, aluminum base alloys are formulated to include specific percentages of zinc, magnesium and titanium as essential alloying ingredients. In a separate form, the invention contemplates the provision of zirconium and boron in place of titanium, or zirconium alone in place of titanium. The instant invention excludes significant amounts of chromium, silicon, iron and other elements such as tin, lead, copper and manganese, all of these being considered undesirable when present beyond the limits to be hereinafter described.

As the following examples will illustrate, the inclusion of the alloying elements of this invention provides results 3,322,533 Patented May 30, 1967 which permit the formation of aluminum castings having properties which greatly exceed conventional casting properties. The exclusion of the alloying elements referred to above greatly contributes to the results of this invention. As will appear hereinafter, other non-essential alloying elements, particularly beryllium, are preferably included within alloys falling Within the scope of this invention. These additional alloying elements are considered beneficial since they can improve the properties of the cast articles rather than being detrimental thereto as is the case with the ingredients which are specifically excluded.

The alloys of this invention consist of the alloys falling within the range of compositions covered in the following table:

TABLE I Alloying element Percent by weight Magnesium .55.95 Zinc 5.5-8.5 Titanium 0.070.3 Aluminum Balance Alternatively, alloys falling within the following range of compositions are contemplated:

TABLE II Alloying element Percent by weight Magnesium .55-.95 Zinc 5.5-8.5 Boron (and/or) .0005-.002 Zirconium .05.25

Elements such as chromium are not included in the above alloys beyond trace amounts. Specifically, chromium cannot be present in an amount exceeding 0.02 percent by weight. Residual elements including tin, lead, copper and manganese cannot be included in amounts exceeding 0.25 percent by weight of the alloys. In addition, any one of these latter elements cannot be present in an amount greater than 0.25 percent by weight.

Beryllium is advantageously added to the alloys of this invention in amounts between .0005 and .03 percent by weight. The beryllium is added to reduce surface tension and it assists in holding down inclusions.

The following provides an illustration of the alloy compositions which are considered to fall within the preferred range of compositions covered by this invention:

TABLE III Alloying elements Percent by we1ght Zinc 6.08.0 Magnesium 0.60.8 Titanium 0.070.2 Boron Up to 0.005 Zirconium Up to 0.25 Aluminum Balance It will be appreciated when considering the Table III that zinc, boron and zirconium can be employed in combinations as well as alternatively in accordance with this invention.

The limits relative to the elements excluded from the compositions of this invention are the same as described above when considering the preferred form of this invention. In all cases, iron should be excluded from the alloy compositions, and, where present, the amount o iron should not exceed 0.15 percent by weight.

The criticality of the elements included within the compositions of this invention can best be demonstrated by reference to tests performed on different related compositions. Employing corresponding alloys, tests have revealed that the amount of zinc in the alloys could be varied between 5.5 and 9.5 percent by weight while the properties desired were still achieved. The most significant improvement obtainable involved alloys containing between 6 and 8 percent zinc.

Similar determinations were made relative to the significance of magnesium contents. When the amount of magnesium exceeded 0.95 percent, the percent elongation of the test specimens dropped oif to a significant degree. Similarly, amounts of magnesium below 0.55 percent provide specimens having significantly lower ultimate and yield strengths. The most desirable combination of strength and elongation was observed with specimens containing between 0.6 and 0.8 percent by weight magnesium.

In the case of titanium, the changes in properties were not as abrupt insofar as the upper limit of 0.3 percent is concerned. However, significant improvements could not be realized with amounts of titanium in excess of this figure. It was apparent, however, that at least 0.07 percent titanium should be employed for achieving maximum improvement. Similar results are recognized when boron or boron and zirconium are utilized alone or in combination with the titanium.

Significant investigations can also be made to demonstrate the effect of the elements which are excluded from the compositions of this invention. Utilization of these ele ments in amounts greater than the limits herein set forth provided in many cases a clear departure from the improved results otherwise obtained. A drop-off in the ultimate strength and in the elongation represented the most significant detrimental effects. It will be apparent that these two properties represent the most important factors when the production of aluminum base castings having high strength is being considered.

One of the important aspects of alloys falling within the scope of this invention relates to the ability of the alloys to self-age in the as-cast condition. The increase in strength which is accomplished by this self-aging is quite significant and is, therefore, a material consideration when determining particular applications for the alloys.

The self-aging characteristics of the alloys can be illustrated by reference to the following example:

Example 1 Alloying elements Percent by weight Zinc .2 Magnesium 0.9 Titanium 0.15 Boron 0.001 Zirconium 0.08 Silicon 0.07 Iron 0.05 Aluminum 1 Balance 1 With less than 0.75 residual metals and impurities.

With an alloy of this type, an ultimate strength of about 33,000 p.s.i. can be achieved on the day cast. An increase in strength of about 5,000 p.s.i. can be achieved after one week and a similar increase is achieved during the second week. When fully aged, an ultimate strength of about 45,000 p.s.i. can be reached. Similar increases are recognized in the yield strength while an elongation of at least two percent can be maintained after stabilization. It is emphasized that the properties can be realized without any heat treatment whatsoever. Furthermore, the cast articles can actually increase in strength during their use if the castings are put into use shortly after their formation.

It is also important to note that the alloys of this invention are adapted to be subjected to solution heat treatment followed by quenching and elevated temperature aging. The properties which can be achieved in alloys of this type are extremely satisfactory and the additional expense of heat treatment is warranted in view of the additional applications to which the alloys can be put. The following examples illustrate this aspect of the instant 1nvention:

Example 2 Alloying elements 1 Plus minor impurities,

In the treatment of these alloys, the sand cast materials are solution heat treated for 6 hours at 1100 F. followed either by a delayed quench in water maintained at about F., or by cooling in still air outside the furnace. Artificial aging is undertaken by heating for five hours at a temperature of about 260 F. Test bars average an ultimate strength of about 46,250 p.s.i., a yield strength of about 43,000 p.s.i. and an elongation of about 6.5 percent. These typical properties are considered exceptional insofar as aluminum castings are concerned.

The user has the option of eliminating the water quench altogether in the above treatment and allowing the material to cool in still or circulating air. This practice will only slightly lower the aforementioned properties. In fact, the high strenght and substantial elongation, achieved with no water quench, provides a highly desirable casting as locked up tensile stresses resulting from the quench are avoided.

Example 3 Alloying elements: Percent by weight Zinc 8.0 Magnesium 0.75 Boron 0.001 Iron 0.05 Silicon 0.08 Tin max 0.001 Lead max 0.001 Copper max 0.25 Manganese max 0.10 Aluminum Balance 1 Plus minor impurities.

Alloys corresponding to the above can be solution heat treated, quenched or not quenched and aged in substantially the same manner as referred to in Example 2. An average ultimate strength of about 51,500 p.s.i., yield strength of about 47,000 p.s.i. and elongation of about 4.8 percent can be achieved with these alloys.

From the preceding examples and discussion, it will be obvious that novel alloys for the production of aluminum castings have been disclosed. The extremely high strength of these alloys in combination with substantial elongation represent clear improvements over prior art compositions. The ability of the alloys to self-age as well as to be subjected to heat treatment comprise features of substantial value since the alloys can be employed for a wide variety of applications. In addition, the option of eliminating the water quench is desirable from a practical production standpoint as well as allowing castings of extreme strength and ductility to be made free from the dangerous locked-up tensile stresses resultant from a rapid quench.

The common heat treatable aluminum casting alloys must be water quenched within 30 seconds of their solutron heat treatment or their full properties will not be developed. When a casting is quickly quenched in water, the thin sections will cool more quickly than the heavier sections. In quenching from 1000 F. castings Wlll contract about .100 inch per foot. With thin sections contracting at this high rate relative to thicker sections, locked-up tensile stresses, often equivalent to or higher than the yield point are developed. As a result of this differential in contraction, the casting will move plastically and warp. Castings with such high locked-up tensile stresses will also fail when subjected to fatigue endurance loadings, for the locked-up tensile stresses detract from the fatigue endurance limit of aluminum alloys. Furthermore, castings so treated cannot successfully be used for sensitive parts requiring a high degree of dimensional stability, as in gyroscopes or electronic components. Thus, the locked-up tensile stresses relieving themselves over a period of time will make the casting dimensionally unstable.

Unlike the common heat treatable alloys, the present invention retains a state of solid solution from the solution heat treatment without the use of a rapid quench. Hence, castings of great strength can be made free from dangerous lcked-up tensile stresses.

There is a growing demand for a construction material in the computer, guidance equipment, and electronics field where dimensional stability on the order of one part per million is required. Any cast aluminum alloy stress relieved at around 700-900 F. and slowly cooled in the furnace will have such a low level of locked-up stresses as to be dimensionally stable to this degree; however, in the common aluminum casting alloys, such as A-356, this stress relieving treatment will anneal the casting very soft and properties will be:

Ultimate strength p.s.i 18,000 Yield strength p.s.i 8,000 Hardness (500 KG BHN) 35 Most importantly in this condition, the hardness of the alloys is so low that they are virtually unmachinable, and the ten-sile and yield strengths are so low as to almost eliminate their use as construction materials.

The alloy covered in this application can be so stress relieved and furnace cooled to achieve the ultimate in freedom of locked-up stresses and then hardened by heating at 310 F. for 15 hours whereupon properties as follows are developed:

6 Ultimate strength p.s.i 318,000 Yield strength p.s.i 30,000 Hardness (500 K-G BHN) 80 In this condition, the machinability of the alloy is excellent and essentially the ultimate in dimensional stability has been achieved with no sacrifice in mechanical properties.

The alloys of this invention also exhibit significant improvements in fluidity and in electrical conductivity when compared to alloys of the type described in applicants Patent No. 2,993,783. Furthermore, the alloys of this invention are available at lower cost.

It will be understood that various changes and modifications may be made in the alloys described which provide the characteristics of this invention without departing from the spirit thereof particularly as defined in the following claims.

That which is claimed is:

1. An aluminum base casting alloy consisting of from 5.5 to 8.5 percent by Weight zinc, from 0.55 to 0.95 percent by weight magnesium, at least one of the members of the group consisting of titanium in an amount from 0.07 to 0.2 percent by weight, boron in an amount from 0.0005 to 0.002 percent by Weight, and zirconium in an amount from 0.05 to 0.25 percent by weight, and the balance aluminum, chromium being excluded from said a1- loy so as not to exceed 0.02 percent by weight, silicon being excluded from said alloy so as not to exceed 0.15 percent by weight, iron being excluded from said alloy so as not to exceed 0.15 percent by weight, and other impurities comprising less than 0.75 percent by weight of said alloy.

2. An aluminum base casting alloy in accordance with claim 1 including from 0.0005 to 0.03 percent by weight beryllium.

References Cited UNITED STATES PATENTS 2,993,783 7/ 196 1 Martin -146 DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner. R. O. DEAN, Assistant Examiner, 

1. AN ALUMINUM BASE CASTING ALLOY CONCICING OF FROM 5.5 TO 8.5 PERCENT BY WEIGHT ZINC, FROM 0.55 TO 0.95 PERCENT BY WEIGHT MAGNESIUM, AT LEAST ONE OF THE MEMBERS OF THE GROUP CONSISTING OF TITANIUM IN AN AMOUNT FROM 0.07 TO 0.2 PERCENT BY WEIGHT, BORON IN AN AMOUNT FROM 0.005 TO 0.002 PERCENT BY WEIGHT, AND ZIRCONIUM IN AN AMOUNT FROM 0.05 TO 0.25 PERCENT BY WEIGHT, AND THE BALANCE ALUMINUM, CHROMING BEING EXCLUDED FROM SAID ALLOY SO AS NOT TO EXCEED 0.02 PERCENT BY WEIGHT, SILICON BEING EXCLUDED FROM SAID ALLOY SO AS NOT TO EXCEED 0.15 PERCENT BY WEIGHT, IRON BEING EXCLUDED FROM SAID ALLOY SO AS NOT TO EXCEED 0.15 PERCENT BY WEIGHT, AND OTHER IMPURITIES COMPRISING LESS THAN 0.75 PERCENT BY WEIGHT OF SAID ALLOY. 